1. Field of the Invention
This invention concerns an output stage control circuit.
2. Description of the Prior Art
Various equipments, for instance, driving motors, illumination equipments, warming units, etc mounted on a vehicle are driven by car batteries. They constitute the battery load and have to be controlled variably depending on the change of the situations, etc.
For satisfying such demands, there has been known an output stage control circuit as shown in FIG. 3, in which a load 21 is a motor, lamp, heater or the like and driven by a transistor 23 at the output stage connected in series therewith.
A constant voltage source 25 producing a constant voltage V is disposed between a DC voltage (+Vcc) supplied from the battery and the ground, and a variable resistor 27 and a resistor 29 is connected in series with the constant voltage source 25, the resistor 29 being grounded.
A control circuit 30 for controlling the output stage transistor 23 comprises a comparator 31, a resistor 33 and a capacitor 35. The comparator 31 is connected at the inverted input terminal thereof to a common junction point 41 between the variable resistor 27 and the resistor 29. Further, the non-inverted input terminal of the comparator is grounded to the earth by way of a capacitor 35 on one hand and connected also to a common junction point 43 between the load 21 and the output stage transistor 23 by way of a resistor 33 on the other hand.
In the output stage control circuit of such a constitution, a reference voltage Vr is set to the comparator 31 as shown in FIG. 4 and the comparator compares to judge a higher level Vh of an input voltage Vin (Vin&gt;Vr) and a lower level V1 of the input voltage Vin (Vin&lt;Vr) relative to the reference voltage Vr. Then, when the reference voltage Vr is set to a higher value and if the input voltage Vin is higher than V1, a voltage at "H" level is outputted from the comparator 31 to render the output stage transistor 23 conductive. Accordingly, a current i.sub.o is supplied to the load 21. When the current i.sub.o flows, the input voltage Vin is lowered for a time T1 determined by the ratio Vh-V1 (=1 mV)/Vh. While on the other hand, when the input voltage Vin is lowered to the V1 level, since the output of the comparator 31 turns to the "L" level, the output stage transistor 23 is switched to the OFF state to interrupt the supply of the current i.sub.o to the load 21. Accordingly, the current i.sub.o is supplied intermittently for a period of time T1.
Further, also in the case of setting the reference voltage Vr to a lower value, the comparator 31 repeats the identical operation. In this case, since the comparison level V'h (&lt;Vh) and V'1 (&lt;V1) are entirely lowered, the output period for the voltage at "H" level of the comparator 31 changes to T2.
As has been described above, the current i.sub.o is supplied intermittently to the load 21 in the conventional output stage control circuit using the comparator 31. However, in the case of intermittently supplying the current i.sub.o to the load 21, if a motor is connected as the load, the output operation such as rotating torque of the motor becomes instable and the motor often generates noises. In addition, since starting or flickering is repeated if the motor or the lamp is connected as the load 21, the working life is shortened.